The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for manufacturing a semiconductor device, which can increase the current drivability of a PMOS transistor.
As semiconductor devices becomes more highly integrated, the length of the channel of a transistor decreases, and the doping concentrations of a source region and a drain region increase. As the channel length of the transistor decreases and doping concentrations increase, a charge sharing phenomenon between the source area and the drain area increasingly would be more likely to occur, degrading the controllability of the gates. As a result an abrupt decrease in threshold voltage known as a short channel effect occurs. The short channel effect if problematic as it deteriorates the drain-induced barrier lowering (DIBL) characteristic and the current drivability of a transistor, and therefore decreases the operation speed of a semiconductor device. A semiconductor device having the conventional planar channel is limited in overcoming the various problems caused by the high integration of a semiconductor device, such as a short channel effect, the deterioration of an operation speed, and so on.
To solve these problems, efforts were made in the past to find ways to enlarge a channel region, by which a recess gate for increasing the effective channel length and a fin gate for increasing the effective channel width became known.
To increase the current drivability of a transistor, the techniques for decreasing the thickness of a gate insulation layer or the depth of a junction area or a super-steep retrograde (SSR) were studied. Also, in order to increase the current drivability of a transistor, efforts were made to find ways to apply stress to a semiconductor substrate.
The directions of stresses, that must be applied to a semiconductor substrate to increase the current drivability of a transistor, vary depending upon the kind of the transistor. Hereafter, the directions of stresses, which must be applied depending upon the kind of a transistor to increase the current drivability of the transistor, will be described with reference to FIGS. 1A and 1B and Table 1.
TABLE 1NMOSPMOSX directiontensile stresscompressive stressY directiontensile stresstensile stressZ directioncompressive stresstensile stress
Referring to FIGS. 1A and 1B, a transistor is formed by forming a gate 110 on a semiconductor substrate 100, and forming source and drain regions 120 in the semiconductor substrate 100 on both sides of the gate 110. The channel length direction of the transistor is denoted as the X direction, the channel width direction of the transistor is denoted as the Y direction, and the height direction of the gate 110 is denoted as the Z direction.
Referring to Table 1, the current drivability and an operation speed of an NMOS transistor can be increased when tensile stresses are applied in the X direction and the Y direction and compressive stress is applied in the Z direction. The current drivability and an operation speed of the PMOS transistor can be increased when compressive stress is applied in the X direction and tensile stresses are applied in the Y direction and the Z direction.
For example, the NMOS transistor can be realized by forming a SiGe layer and a Si layer sequentially on a semiconductor substrate made of bulk silicon through epitaxial growth and then forming NMOS gates on the Si layer. In this case, since the Si layer is formed on the SiGe layer, the distance between lattices in the Si layer increases because the SiGe layer has greater distance between lattices then the Si layer. By forming the Si Layer and the SiGe layer sequentially as described above, tensile stresses can be applied in both the channel length direction and the channel width direction of the NMOS transistor, that is, the X direction and the Y direction. Hence, the current drivability and the operation speed of the NMOS transistor can be increased.
In the conventional art, the same kind of stresses, that is, tensile stresses are applied in the channel length direction and the channel width direction of the NMOS transistor. However, in the case of the PMOS transistor in which different kinds of stresses must be applied in the X direction and the Y direction to increase the current drivability, the current drivability cannot be properly increased through the method of applying stresses to the semiconductor substrate in the same manner as the NMOS transistor. In the case of the PMOS transistor, to increase the current drivability compressive stress must be applied in the X direction and tensile stress must be applied in the Y direction, and as such, the method of applying the tensile stresses in both the X direction and the Y direction cannot be used.